Methods and apparatus which utilize ultrasound energy for the diagnostic imaging of body structures are well known. Typically, ultrasound energy is directed into and scattered from body tissues. The amplitude of the resultant echoes is detected and displayed to form an image which characterizes the scattering structures. Virtually all commercial medical ultrasound imaging systems form images from the envelope of echoes which are reflected from an interface of tissue types such have different acoustic impedances. The images are, therefore, useful to delineate the outlines of various body organs and lesions. However, because the image makes no use of the phase information in the echo signals, it is not generally possible to identify tissues types.
The prior art teaches a number of ultrasound systems which are capable of identifying tissues having specific characteristics. U.S. Pat. No. 4,389,893 to by OPHIR and MAKLAD is typical of a class of apparatus which attempts to characterize tissue types by measuring local ultrasonic attenuation. Likewise, U.S. Pat. No. 4,270,546 to PERILHOU and COURSANT describes apparatus for identifying the directional characteristics of tissue structures.
The ultimate aim of tissue characterization apparatus and methods is, of course, to differentiate and identify the tissues of the various body organs as well as the pathology of healthy and diseased tissues to provide a remote, noninvasive biopsy. Although the methods and apparatus of the prior art may have some use in differentiating particular tissue characteristics, they have not yet attained these goals.